It is well known that the Faraday effect in magneto-optic materials can be used to provide a non-reciprocal device that can serve as an isolator, i.e., a device that permits passage of light in only one direction. Among known magneto-optic materials are Bi-substituted rare earth iron garnets such as (Bi.sub.x Tb.sub.1-x).sub.3 (Fe.sub.y Ga.sub.1-y).sub.5 O.sub.12. For a description of magneto-optic isolators see, for instance, S. Makio et al., Electronics and Communications in Japan, Part 2, Vol. 74(2), p. 323 (1991), incorporated herein by reference.
Magneto-optic materials used in prior art isolators typically have relatively high saturation magnetization, which typically requires the use of a relatively large, high field permanent magnet (e.g., SmCo) that typically is not only expensive but also may affect, and/or be affected by, nearby components.
It is well known that ferrimagnetic garnet materials generally have magnetic domains, and that application of a magnetic field to such a material results in change of the domain structure. In particular, in a field greater than or equal to the saturation field, such a material generally will contain essentially a single magnetic domain, with magnetization oriented along the direction of the applied field. Upon removal of the applied field the material generally reverts to a randomly oriented multi-domain state. In magnetically saturated material the Faraday rotation experienced by light transmitted through the material is also saturated. Optical isolators generally are designed to operate with magnetically saturated magneto-optic material, although unsaturated material also has utility.
It would clearly be advantageous to have available magneto-optic materials that have relatively low saturation magnetization and are otherwise suitable for use in optical isolators. It would be particularly desirable to have available a magneto-optic material that can, after having been magnetically saturated, substantially remain in the saturated (single domain) state. Those skilled in the art will recognize that such a material for instance would make possible isolators without (or with relatively weak) permanent magnets. This application discloses materials that have the above described property, which herein will be referred to as "latching". K. Matsuda et al., Applied Physics Letters, Vol. 59(5), p. 507 (1991), propose and demonstrate a magnetless Faraday rotator that utilizes a garnet waveguide with stripe magnetic domains.
Bi-doped rare earth iron garnets are known (see, for instance, Japanese patent application Heisei 3-306697 of Oct. 24, 1991 by Y. Toba), and are used in optical isolators. These garnets generally have values of saturation magnetization above about 150 G. For instance, some commercially available (Bi,Tb).sub.3 (Fe,Ga).sub.5 O.sub.12 materials have values in the range 150-390 G, and some commercially available (BiGd).sub.3 (Fe,Ga,Al).sub.5 O.sub.12 materials have values of about 200 G. Garnets that have much lower values of saturation magnetization are also known. For instance, G. Winkler, "Magnetic Garnets", Vieweg, Germany (1981), p. 611, discloses material of composition Er.sub.2 Eu.sub.1 Fe.sub.3.7 Ga.sub.1.3 O.sub.12 with a saturation magnetization of 10 G. The material was considered to have utility for a magneto-optic display. P. Hansen et at., IEEE Trans. Magn., MAG-20, p. 1099 (1984) disclose a low moment material substantially of composition Bi.sub.1 Gd.sub.2 Fe.sub.4.4 Ga.sub.0.2 Al.sub.0.4 O.sub.12 for thin film visible light displays using the compensation point of the material and thermomagnetic writing.
It will be understood that, in order to be useful for isolator application, a magneto-optic material not only needs to have relatively high specific Faraday rotation and relatively low saturation magnetization but typically also has to have low attenuation (e.g., &lt;0.2 dB for a 45.degree. film) for, typically, radiation of the signal wavelength (e.g., 1.5 .mu.m). Furthermore, it will generally be necessary that the material can be grown as a single crystal film that is thick enough to provide 45.degree. rotation per pass of the relevant radiation through the saturated film. However, other possible applications of latching magneto-optic material may not demand all of these features.